(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to increase the performance of complementary metal oxide semiconductor (CMOS), devices, via the use of a channel region comprised in part with a strained silicon-germanium layer
(2) Description of Prior Art
An attractive approach for increasing CMOS transistor performance is the use of a silicon-germanium (SiGe), layer located in the channel region of the CMOS device. The biaxial stress in the SiGe layer, creating strain induced band structure modification, allows enhanced transport properties of electrons for N channel (NMOS), devices, and enhanced transport properties of holes for P channel (PMOS), devices, to be realized. However the ability to place a SiGe layer in a region that will subsequently underlay the gate structure, as well as to subsequently accommodate shallow source/drain regions, can be challenging in terms of integrating the SiGe devices into a conventional CMOS process.
This invention will teach a process in which a SiGe layer is selectively grown on only the active device regions, eliminating the need for costly patterning procedures to remove portions of the SiGe layer overlying non-active device regions, such as insulator filled shallow trench regions. This invention will also describe a composite layer comprised with silicon layers overlying as well as underlying the SiGe component of the composite layer. The silicon layer overlying the SiGe layer, silicon cap layer, allows a gate insulator layer to be thermally grown consuming only a portion of the silicon cap layer. In addition, this invention will also describe the thickness and composition of the silicon cap, SiGe and silicon buffer layers, needed to accommodate the biaxial compressive strain needed for enhanced hole mobility in the PMOS channel region, and the strain induced energy splitting in the conduction band allowing enhanced electron transport properties to enhance electron velocity overshoot in the channel region of NMOS devices. Prior art, such as Ismail et al, in U.S. Pat. No. 5,534,713, describe the use of thick buffer layers, used with a SiGe layer, however the use of these thick layers may not allow for the easy integration into a conventional CMOS process as this present invention does.